Fig. 1: Digonal ring closures for the synthesis of azetidines.

a Original and revised Baldwin’s rules for digonal ring closures. b Rare examples of radical 4-exo-dig and 5-endo-dig cyclizations. c Reaction design: radical 4-exo-dig cyclization of ynamides triggered by visible-light activation with a photoredox catalyst (PC). d Representative examples of naturally occurring/biologically relevant azetidines.